The sphingomyelin pathway is a ubiquitous, evolutionarily conserved signaling system initiated by hydrolysis of sphingomyelin to generate the second messenger ceramide. Two forms of sphingomyelinase, distinguishable by the pH optima, are capable of initiating signaling. Acid sphingomyelinase (pH optimum 4.5-5.0) was originally identified as a lysosomal hydrolase required for turnover of cellular membranes (for review see Kolesnick, R. N., 1991, Prog. Lipid Res. 30 1-38). However, Kronke and co-workers proposed that this enzyme was also targeted to the plasma membrane and signaled in response to activation of the 55 kD TNF receptor (Wiegmann, K. et al., 1994, Cell 78:1005-1015). Activation of acid sphingomyelinase has also now been associated with signaling via Fas, CD28 and the interleukin (IL)-1 receptor (Cifone, et al., 1993., J. Exp. Med. 177:1547-1552; Boucher, L.-M. et al., 1995, J. Exp. Med. 181:2059-2068.; Liu. P. & Anderson, R. G. W., et al., 1995, J. Biol. Chem. 270:27179-27185. Human acid sphingomyelinase is the product of a single gene, although alternative processing of the primary transcript allows for the generation of multiple forms (Schuchman, E. H., et al., 1991, J. Biol. Chem. 266:8531-8539; Schuchman, E. H., et al., 1992, Genomics 12:197-205. Inherited mutations of the human acid sphingomyelinase gene lead to enzyme deficiency and the genetic disorder known as Niemann Pick disease (NPD; Brady et al., R. O., et al., 1966, Proc. Natl. Acad. Sci. USA 55, 366-369.; Schneider, P. B. & Kennedy, E. P., 1967, J. Lipid Res. 8:202-209.
Neutral sphingomyelinase (pH optimum 7.4) was originally defined as a Mg2.sup.+ -dependent enzyme localized to the outer leaflet of the plasma membrane (Rao, B. G. & Spence, M. W., 1976, J. Lipid Res. 17:506-515.; Yedger, S. & Gatt, S., 1976. Biochemistry 15:2570-2573. However, a Mg2.sup.+ -independent isoform of neutral sphingomyelinase which localizes to the cytoplasm has recently been identified (Okazaki, T. et al., 1989, J. Biol. Chem. 264:19076-19080.; Okazaki, T. et al., 1994, J. Biol. Chem. 269:4070-4077.). The neutral sphingomyelinase has not yet been characterized at the molecular level.
Neutral sphingomyelinase activation has been demonstrated in response to cellular stimulation with TNFa (Wiegmann, K. et al., 1994, Cell 78:1005-1015) anti-Fas antibody (Tepper, C. G., et al., 1995, Proc. Natl. Acad. Sci. USA 92:8443-8447.; Cifone, M. G. et al., 1995, EMBO J. 14:5859-5868.), and vitamin D (Okazaki, T. et al., 1989, J. Biol. Chem. 264:19076-19080; Okazaki, T. et al., 1994, J. Biol. Chem. 269:4070-4077.). It has also been suggested that neutral sphingomyelinase signals in response to IL-1b (Mathias, S. et al., 1993, Science 259:519-522) and ionizing radiation (Halmovitz-Friedman, A. et al., 1994, J. Exp. Med. 180:525-535.).
Although ceramide has been implicated as the second messenger for a variety of stress stimuli including TNFa, Fas ligand, ionizing radiation, heat shock, ultraviolet light and oxidative stress (Obeid, L. M. et al., 1993, Science 259:1769-1771.; Cifone, M. G. et al., 1993, J. Exp. Med. 177:1547-1552.; Jarvis, W. D., et al., 1994, Proc. Natl. Acad. Sci. USA 91:73-77; Fuks, Z., et al., 1994, Cancer Res. 54:2582-2590; Halmovitz-Friedman, A., et al., 1994, J. Exp. Med. 180:525-535.; Gulbins, E. et al., 1995, Immunity 2:341-351.; Verheij, M. et al., 1996, Nature 380:75-79; Jarvis, W. D. et al., 1996, Clin. Cancer Res. 2:1-6). Evidence for such speculation has been circumstantial (Verheij, M. et al., 1996, Nature 380:75-79; Hannun, Y. A. & Obeid, L. M, 1995, Trends Biochem. Sci. 20:73-77). Definitive proof, therefore, that ceramide generation is a primary mediator of the apoptotic response is lacking.